Roll-up doors are commonly found on various static structures, such as garages and industrial buildings, and on cargo-carrying truck and trailer bodies.
Such doors typically include a plurality (i.e., two or more) of series-connected horizontally-elongated rectangular panels mounted on rollers for guided upward and downward articulated movement along a pair of horizontally-spaced inverted L-shaped tracks on either side of an access opening. The door is arranged to be selectively moved between a lowered position, in which such panels are vertically arranged to close the access opening, and a raised out-of-the-way position, in which such panels are disposed horizontally overhead to permit ingress and egress through the opening.
The weight of the door is, of course, related to the number, size and particular construction of the panels. Some panels are simply formed of wood. Other panels are formed as a tubular metal extrusion, and may be filled with some other material. For example, if used to close the access opening of a refrigerated trailer, such panels may be filled with foam to provide a thermally-insulated door. Still other doors have a metal skin secured to a plywood core.
In any event, it is common to use a spring balancer between the door and the support on which it is mounted. Such balancer generally acts as a counterbalance to prevent a person from having to lift the entire weight of the door, and from having to slow its descent. To this end, it is known to employ one or more torsional springs acting between an the door and the support. Typically, each spring surrounds a portion of a shaft which is journalled on the support. Two cables are connected to the door and are arranged to be wound about two drums mounted fast to the shaft when the door is raised. These drums and cables are generally arranged to support about one-half of the nominal load, which is a function of the weight of the door. This load may, for example, be about thirty pounds [13.6 kg] less than the weight of the door itself. Hence, when the door is raised or lowered, an operator need only supply sufficient additional incremental energy which, when coupled with the energy given up or absorbed by the spring, will overcome the weight of the door. In other arrangements, the spring may be wound more tightly so that the door will lift automatically, when released, without the operator having to supply any additional energy. Indeed, if desired, the spring may be wound even more tightly so that the operator must supply negative energy (i.e., opposing the lifting force of the unwinding spring) to prevent the door from lifting too quickly. In any event, in a conventional "two-spring two-cable" spring balancer, each spring and each cable typically supports about one-half of the load. Hence, if one spring or one cable breaks, its surviving mate may not possess sufficient energy to hold the partially-opened door, particularly if it is not fully overhead. This may possibly cause a significant safety hazard to persons working beneath the door, such as persons loading or unloading a truck or trailer.
Accordingly, there is believed to be a need for an improved spring balancer, which is particularly suited for, but not limited to, use with a roll-up door, and which affords an additional measure of safety in the event that a spring or cable should break.